In a conventional mobile communication system, telecommunications carriers providing the service previously determine the target cell coverage of each radio base station, and transmission parameters are set for each of the radio base stations such that cell coverage can be obtained. The transmission parameters are, for example, radio transmission electrical power with respect to each cell of a radio base station, the height of antennas, antenna patterns (or beamforming), and tilt angles. When the system is in operation, there is a significant variation in traffic volume depending on, for example, the time zone. Consequently, it is inefficient if all the radio base stations in a predetermined area are always in operation or if a transmission parameter is always set to a value that ensures wide cell coverage with respect to all of the radio base stations.
Related to the above points, there is a known mobile communication system that is designed with the aim of reducing electrical power consumption by changing the zone configuration of a radio base station in accordance with the communication traffic state of the system and of preventing a communication area being unavailable due to a change in the zone configuration (see Japanese Laid-open Patent Publication No. 10-145842). In this mobile communication system, a radio control station includes a unit for monitoring, in a centralized manner, communication traffic reported from a radio base station group; a unit for monitoring, in a centralized manner, the electric field intensity of a main radio base station that is reported from each sub radio base station in the radio base station group; a unit for transmitting a radio output variable control signal to a radio base station in the radio base station group; and a database that manages the arrangement configuration of the main radio base station and a plurality of sub radio base stations.
In the conventional mobile communication system described above, if the communication traffic in a predetermined service area that includes a plurality of radio base stations is below a threshold, the zone configuration is changed, i.e., a control is performed such that the radio transmission function in a specific radio base station is stopped and radio transmissions output from another radio base station are increased. However, with such a control, when a handover of a mobile station is performed between cells formed by the radio base stations, there is a problem in that the mobile station sometimes does not maintain the communication in the state it was in before the handover. This problem will be described below with reference to FIGS. 1A, 1B and 2.
FIG. 1A illustrates a mobile communication system in which a plurality of radio base stations eNBs 10 to 16 are arranged in a predetermined service area. Each of the radio base stations is a communication device that forms a cell. In FIG. 1A, all of the radio base stations in the service area are in a normal operation state. In contrast, in FIG. 1B, the radio base stations eNBs 11 to 13 remain in the normal operation state; however, the radio base stations eNBs 10, 14, and 16 are in an electrical power saving state. Furthermore, in FIG. 1B, the radio base station eNB 15 is in a state in which transmission electrical power is higher than that in the normal operation state; therefore, the cell formed by the radio base station eNB 15 covers the cells that have been formed by the radio base stations eNBs 10, 14, and 16 when they were in the normal operation state. Referring to FIG. 1B, a description will be given of a case in which a handover is performed on a mobile station UE from a cell formed by the radio base station eNB 12 to a cell formed by the radio base station eNB 15.
In the cell configuration illustrated in FIG. 1B, the cell area to be covered by the radio base station eNB 15 is enlarged; however, because neither the signal processing capacity performed by the radio base stations nor the number of mobile stations that can be accommodated by the radio base stations is changed before and after a change in state, hence, there may be a case in which a mobile station UE that has just been handed over is not accommodated by a radio base station. In such a case, if a handover is performed within a predetermined service area, the total volume of the communication traffic in the service area does not change; therefore, the conventional mobile communication system described above does not perform a control in which the cell configuration (i.e., the zone configuration) is changed. Specifically, a control for changing the cell configuration from the state illustrated in FIG. 1B to that in FIG. 1A is not performed; therefore, the signal processing capacity in accordance with the handover of the mobile station UE is not improved.
Furthermore, even if a control for changing the cell configuration from the state illustrated in FIG. 1B to that in FIG. 1A is performed on the basis of the detection of the handover of the mobile station UE, it takes a certain amount of time to change the cell configuration; therefore, the mobile station UE does not maintain the communication state that it had before the handover immediately after the handover. Specifically, communication may possibly be stopped after the handover of the mobile station UE or the transmission speed may possibly be reduced. This problem will be described with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B illustrate, along the time axis, the above-described problem related to the handover of the mobile station UE. In FIG. 2A, the state of the radio base station eNB 10 is illustrated along the time axis. In FIG. 2B, the transmission speed of the mobile station UE is illustrated along the time axis. In FIGS. 2A and 2B, if a time period (time t0) has elapsed since a handover (HO) and if a mobile station UE is handed over to the radio base station eNB 15, the radio base station eNB 15 does not accommodate the mobile station UE in terms of the signal processing capacity performed by the radio base station eNB 15; therefore, it is determined that the cell configuration is changed from the state illustrated in FIG. 1B to that in FIG. 1A. However, because it takes a certain amount of time to reach time t1 at which the cell configuration is changed, the transmission speed of the mobile station UE is reduced between time t0 and time t1 (S1→S0) or communication is stopped. The possibility of the occurrence of a reduction in the transmission speed of the handed over mobile station UE or the occurrence of the stopping of communication as described above is high in a future mobile communication system, such as Long Term Evolution (LTE), Ultra Mobile Broadband (UMB), and worldwide interoperability for microwave access (WiMAX), because calls with high bit rates are processed.